The present invention relates to electrically tunable microlens arrays prepared by pattern polymerization of photopolymerizable mixtures containing liquid crystals. More specifically, the invention relates to polymer dispersed liquid crystals photopolymerized by using multiple-wave mixing.
Liquid crystals have long been utilized in the prior art for their ability to change their optical orientation in the presence of an electric field. As thin films, liquid crystals are widely used in various electro-optical display and control applications such as optical switches, variable transparency windows, and large area flat panel displays. In all these devices, the anisotropic electrical and optical properties of liquid crystals are exploited by switching them between a strong scattering OFF state to a transparent light transmitting ON state using external electric fields.
Polymer dispersed liquid crystal display devices are increasingly desired due to production ease and display brightness. Polymer dispersed liquid crystals (PDLCs) are formed from a homogeneous mixture of prepolymer and liquid crystals. As the polymer cures, the liquid crystals separate out as a distinct microdroplet phase. If the polymer is a photopolymer, this phase separation occurs as the prepolymer is irradiated with light. When a PDLC blend is subjected to an externally applied electric field above a certain value, the liquid crystal molecules are oriented parallel to the field direction. In this oriented state, the film will be transparent to the naked eye since the ordinary refractive index of the liquid crystal can be matched approximately to that of the polymer. When the electric field is turned off, the blend switches back to its scattering turbid state.
Another area of interest for liquid crystal materials is for use in microlens because of their tunable focal length that are crucial for optical beam steering and image processing. The principle of focal length tuning of a microlens is illustrated in FIG. 1. Therein, the microlens is designated generally by the numeral 100, and consists of a liquid crystal droplet 100 to defined by polymer walls 104. The liquid crystal droplet 102 and polymer 104 are contained between substrates of transparent glass 106 having an indium-tin-oxide (ITO) electrode layer 108 thereon. In the absence of an external electric field, the liquid crystal directors 110 are randomly oriented, causing light to scatter. When a voltage is applied across the liquid crystal micro-droplet, as shown in FIG. 1, the liquid crystal directors 110 tend to align toward field direction, e.g., along the droplet curvature near the surface, but tend to become straight near the center. The curvature of the liquid crystal alignment may be altered by the electrical field, which in turn guides the incoming light waves 112 to convert to a focal point 114. In this manner, the focal length can be tuned by controlling the applied voltage.
The liquid crystal microlens, hither to reported, have been fabricated by drilling holes on ITO coded glass electrodes in filling with liquid crystals. Another way of fabricating the microlens is through pattern-photo polymerization of liquid crystal/photo-curable monomer mixtures by either masking with an array of black dots or patterning with parallel electrodes. However, these drilled holes or masking dots or microelectrodes are too large to fabricate microlenses of nano-sizes; most microlens thus produced are at best in the range of a few hundred microns in size. For better image resolution, it is desirable to reduce the microlenses size to a few hundred nanometers or smaller, which motivates the present study.
One drawback to liquid crystal display devices is that the display quality is dependent on the angle of observation. U.S. Pat. No. 5,886,760, to Ueda et al., discloses a microlens array in the liquid crystal display device that widens the viewing angle. The microlens array is formed by polymerizing a transparent resin within the apertures of a mesh-like sheet. The polymerization is accomplished by using an energy beam with a UV ray or an electron beam, or by thermosetting. The size of each microlens is dependent on the mesh size of the sheet, which is limited to 100 to 600 mesh. The limited range of microlens size restricts the resolution which can be achieved with a microlens array formed using a mesh-like sheet.
Other methods which have been used to control microlens size and shape include drilling micropores on glass substrates or masking the UV rays. In these methods, the size of the microlens are on the order of 300 to 500 microns. Smaller microlenses are needed to provide the type of resolution required by applications such as medical imaging, diffraction grating, and beam steering.
Droplet size affects switching voltage, switching speed, and contrast ability.
In general, smaller droplets require higher switching voltage, longer switching times, and provide better contrast. In addition to droplet size, the shape and uniformity of the droplets affects the performance characteristics of the polymer dispersed liquid crystal displays. The arrangement and uniformity of the dispersion of the liquid crystals in the polymer matrix should also be considered. These characteristics of the polymer dispersed liquid crystal are sometimes referred to as the domain morphology. Using conventional radiation curing techniques, illumination of the uncured sample is uniform, and phase separation occurs as a random process in which droplets form throughout the sample. Likewise, thermal polymerization is a bulk curing method in which the whole sample is heated. Droplet size is generally controlled indirectly by, for example, adjusting relative amounts of monomer, chain extender, surfactant, and other components. U.S. Pat. No. 5,949,508 to Kumar et al. discloses a technique which preferentially exposes one side of a cell containing prepolymer material and liquid crystals to UV radiation. This causes a polymer layer to form adjacent to the side nearest the UV light source. By controlling the power, collimation, and exposure time of the UV light, grooves, channels or patterns can be formed which can receive liquid crystal material. Uniformity of size, shape, and position of the liquid crystals is difficult to achieve, but in general, droplet size is on the order of at least 400 micrometers. Non-uniformity contributes to scattering and erodes the optical performance of the liquid crystal display.
U.S. Pat. No. 5,942,157, to Sutherland et al., describes the use of two-wave mixing in the production of volume hologram materials. The PDLC is exposed to coherent light to produce an interference pattern inside the material. This technique produces clear, orderly rows of polymer dispersed liquid crystal having uniform size and shape. These polymer dispersed liquid crystal materials are then useful for recording volume diffraction grating in 1-dimension.
Because the performance and capabilities of polymer dispersed liquid crystal displays are critically affected by the domain morphology of the PDLC, a process is needed to effectively control the liquid crystal droplet size, shape, number, and arrangement within the polymer matrix. Such a process would preferably be reproducible and produce higher yields in fabricating polymer dispersed liquid crystal devices.
For purposes of the present disclosure, by xe2x80x9cmultiple-wave mixingxe2x80x9d or xe2x80x9cmixing of multiple electromagnetic wavesxe2x80x9d it is meant the creation of an interference pattern inside a target material by the use of at least four electromagnetic waves to yield 2- or 3-dimensional (2-D or 3-D) diffraction grating. The term xe2x80x9cphotopolymerizable materialxe2x80x9d is to be understood to encompass either monomers having photopolymerizable functional groups or monomers intermixed with photoinitiator dyes such that the monomers within the mixture will be polymerized upon photoinitiation of polymerization.
The present invention relates to polymer dispersed liquid crystals prepared by the pattern photopolymerization of photoreactive mixtures comprising a photopolymerizable material and liquid crystals, wherein the photopolymerization is characterized by the use of multiple-wave mixing.
The present invention further relates to a process of preparing polymer dispersed liquid crystals, the process comprising the steps of providing a photoreactive mixture comprising a photopolymerizable material and liquid crystals; exposing the mixture to a radiation pattern, wherein the radiation pattern results from the mixing of multiple electromagnetic waves; and polymerizing the photopolymerizable material.
The present invention also relates to an electrically tunable microlens array comprising polymer dispersed liquid crystals prepared by the pattern photopolymerization of photoreactive mixtures comprising a photopolymerizable material and liquid crystals, wherein the photopolymerization is characterized by the use of multiple-wave mixing.